A solid golf ball includes one-piece golf balls comprising one constituent, two-piece golf balls comprising a solid core and a cover covering the former, and multi-layer golf balls having one or more suitable intermediate layers between the solid core and the cover. Of these solid golf balls, there are known those in which a monomer having an unsaturated bond, for example, the metallic salt of an .alpha.,62 -monoethylenic unsaturated carboxylic acid, has been incorporated as a co-crosslinking agent in order to improve both rebound coefficient and impact resistance. These solid golf balls themselves have fairly superior performances, but golf balls having more superior rebound coefficient and durability are required.
Hitherto, the moderate hardneses (compression; compression ratio) and durability of said golf balls, as produced from the composition containing a monomer such as the metallic salt of an .alpha.,.beta.-ethylenic unsaturated carboxylic acid, have been considered to result from the fact that the monomer is grafted to the polybutadiene main chain by the action of a free radical initiator, thereby acting as a co-crosslinking agent. But, when the grafted chain produced in this co-crosslinking becomes too long, a reduction in the rebound performance of the golf ball is brought about in the same manner as in blending a polybutadiene rubber with other polymers.
The present inventors tried to give a moderate hardness and durability to solid golf balls and at the same time remarkably improve their rebound performance by regulating the length of the grafted chain produced in co-crosslinking of the foregoing .alpha.,.beta.-ethylenic unsaturated carboxylic acid, and as a result, found that one group of polysulfide type compounds has a very superior performance as an agent for regulating the molecular weight of the grafted chain. The present inventors thus completed the present invention.